1. Field of the Invention
This invention relates to a method of reflow bonding electronic parts such as large scale integrated circuits (abbreviated as LSI), resistor packages, connectors, etc. on a printed circuit board, and also relates to an apparatus used therefor. More particularly, this invention is intended to ensure reliable bonding between electronic part leads and foot patterns on a printed circuit board. This is especially important when a multi-layer printed circuit board is used, and a printed board assembly is utilized in a large scale information processing system.
2. Description of the Prior Art
The method of reflow bonding of the prior art is explained using FIG. 1. FIG. 1 shows an example of a perspective view of a reflow bonding apparatus. The external appearance of the apparatus is almost similar for both types of the prior art and the present invention. A table 50, on which a printed circuit board (not shown) is fixed, is movable along X and Y axes. A bonding head assembly 51 is provided and it has two reflow bonding tips 52 which plays an important role in the present invention. Each bonding tip has a shape such that an electric current can flow into a horizontal tip portion of a bonding tip 52 and flows out therefrom, and each leg portion is connected to a welding power source 53 and an electric current heats up the horizontal tip portion of bonding tip 52 during a bonding operation.
The apparatus is controlled by switches on an operator control panel 55 and the operation is sequentially controlled by a controller, which is housed in a lower cabinet 54. Further explanations on other accessories which have no direct relation to the present invention are omitted.
FIG. 2 shows schematically an enlarged side view of a tip portion 521 of the bonding tip 52 in the prior art at the time when the bonding head 51 in FIG. 1 moves downward by a drive mechanism attached thereto onto the horizontally bent portion 211 of a lead 21 of an electronic part 2 and presses down the lead on a printed circuit board 1 with a pressurizing force. The bent portion 211 is arranged just on a foot portion 61 of the circuit pattern on a printed circuit board 1.
FIG. 3 shows a side view of the tip portion 521 of bonding tip 52 in the direction of an arrow X in FIG. 2. Tip portion 521 shows that two legs are formed and each leg is connected separately to the terminal of welding power source 53. The electric heating current flows in from one leg portion 521a and flows out to another leg portion 521b, and the horizontal portion 521c is heated up during the specified period of bonding operation, because it is formed such that it has a relatively higher resistance than that of the leg portion. The hatched area 522 is coated with an insulating material to protect the sensitive electronic parts 2 from a damage due to the electric current flow.
The width W of tip portion in FIG. 3 is determined such that it sufficiently covers the width of a plurality of leads 211 aligned in the vertical direction of FIG. 2. The width W and the distance D between two bonding tips in FIG. 2 change depending on the outline dimensions of electronic parts 2, therefore the bonding tips are changed when parts having different dimensions are to be reflow bonded.
As an example in the prior art, FIG. 4 shows schematically a side elevational view of a portion of the bonding head 51 including bonding tips 52, printed circuit board 1 and electronic part 2 to be reflow bonded, with portions broken away for clarity.
Bonding head provides a pusher rod 35 (partly shown), which is movable up and down along the Z direction, and also provides a coiled spring 31 (partly shown) and a guide arm 34 for each bonding tip 52. Bonding tip is fitted in the guide arm 34, and can be movable upward in the Z direction by a coiled spring 31 after it hits against the printed circuit board or leads thereon, and is pushed upward against a force of coiled spring 31. When the pusher rod 35 moves downward by a drive mechanism (not shown) and the horizontal portion 521c of bonding tip touches lead 211 on the printed circuit board 1, coiled spring 31 begins to contract. The pusher rod has a specified stroke of movement, therefore the coiled spring 31 gives a pressurizing force proportional to the contraction amount thereof onto the leads to be reflow bonded. When the movement of pusher rod is completed by a drive mechanism, then the reflow bonding operation starts by flowing electric current from the welding power source 53 shown in FIG. 1.
The above described structure of the bonding head in the prior art has several weak points such that it is difficult to get an uniform pressurizing force onto leads placed on the printed circuit board 1 because of a thickness change of the printed circuit board, or warp thereof and so on. Moreover the lateral contact positions in the X direction of each bonding tip 52 on lead 211 are liable to deviate from the precise expected positions. Consequently the fillet of soldered portion A shown in FIG. 2 is not formed satisfactory.